Some microelectronic packages, e.g. high frequency logic devices, power transistors, diodes and silicon controlled rectifiers need a controlled temperature to work properly. Usually the temperature cannot exceed a predetermined maximum (e.g. 80.degree. C.) which may vary according to the different applications. In the manufacturing of these microelectronic packages it is common practice to inteipose a thermal conductive material, that is called usually "thermal grease", between the chip package and the heat sink, i.e. a piece of highly thermal conductive metal, in order to improve the heat transfer outside the package. The thermal grease is spread on the top of the chip package and the heat sink is then mounted and clamped on it to assure a strong mechanical contact and good heat diffusion. The thermal conductivity of grease is ensured by the presence of a conductive powder (e.g. zinc oxide). A state of the art thermal grease is the TC-208 and TC-228 produced by Thermoset Lord Chemical Products (Indiana, USA), which contains about 50-75% in weight of zinc oxide, in an organic matrix, mostly composed of polyether polyol polymers which can be fluorinated or no-fluorinated depending on the application; fluorinated compounds are the best solution when the temperature must stay below 200.degree. C., while if the temperature should be kept under 80.degree. C. non-fluorinated compounds are preferred.
When a package with a heat sink fails, it needs to be replaced with a new one following this procedure:
1) remove the heat sink; PA1 2) remove the thermal grease from the top of the package; PA1 3) apply a blow of hot air for reflowing the solder alloy connecting the package with the board; PA1 4) rework the module. PA1 1) a possible damage of the fragile components, as for instance the contacts of the connectors which could be twisted during the brushing; PA1 2) the residues being trapped in the smallest spaces of the board, e.g. between pins and solder joints or inside via holes.
The cleaning up of the thermal grease must be performed carefully avoiding any spreading of this product because the contamination of electrical connections, e.g. pads, circuit lines, connectors and test points may compromise the function of the electronic card causing a variation of the electric current that flows through the circuit.
Simple solvents, as for example water, alcohol or water soluble fluxes are not able to flow through the via holes during the cleaning operation and leave some thermal grease in the holes. Moreover the occlusion of the via holes by the thermal grease produces the trapping of the flux inside the holes, with possible corrosion phenomena. In fact thermal grease is not normally completely dissolved by water or alcohol and some residues may remain on the surface of the package after the package has been treated by dipping or spray techniques. For this reason, also a mechanical action (e.g. brushing) is normally needed to completely remove the thermal grease. This mechanical cleaning action is an additional burden and usually requires human intervention. Furthermore, the mechanical operations involves two problems:
On the other hand the use of stronger solvents on the thermal grease, such as basic or acid solutions, could damage the materials of the board and of the electronic components. Therefore these chemicals cannot be used for cleaning assembled electronic cards which always present corrodible materials e.g copper, tin alloys, organic resins.
Therefore an easy chemical treatment which removes the thermal grease without the need of a subsequent mechanical action would be highly desirable.
It is an object of the present invention to provide a technique which alleviates the above drawbacks.